What Goes Behind Siding, and Why Those Layers Keep Your Walls Dry

What goes behind siding matters as much as the siding you see. First comes sheathing, the flat panels nailed to the wall frame. Over that goes a water-resistive barrier, such as house wrap, that sheds water the siding lets past. Flashing at windows, doors and other openings steers water out, and trim pieces hold the siding edges. Together they form a drainage plane, a path that carries stray water down and out of the wall.

Siding is a first line of defense, and some rain always gets past it. Wind can push water up under laps, and wicking can draw it into tight joints. The Building America Solution Center, a U.S. Department of Energy resource, calls for a continuous drainage plane behind siding. Model home building codes also require a water barrier and a way for water to drain out. Those layers decide whether a stray drop dries out harmlessly or rots the wood.

Here is siding underlayment explained one layer at a time. We start with how house wrap and other barriers work, and why a small gap behind siding helps. Then you will see what gets checked in the sheathing and where flashing goes beyond the obvious spots. Last, we cover when you can fix or upgrade these layers, which is only while the siding is off.

House Wrap Under Siding: How the Water Barrier Works

House wrap and other water barriers work like shingles on a roof. Each upper layer laps over the one below, so water running down never finds an open seam. FEMA's coastal building guide calls for horizontal laps of at least 6 inches. It calls for vertical laps of 6 to 12 inches, with the larger laps where wind-driven rain is worse. FEMA also warns that tape and sealant cannot fix a lap that runs the wrong way.

Water barriers come in several types, and each one handles water a little differently. Felt paper, or building paper, soaks up water more easily and handles sun worse than house wrap. Liquid coatings that are rolled or sprayed on and peel-and-stick sheets seal tightly around openings. Some sheathing panels come with a built-in barrier that only needs its seams taped. Foam board with taped joints can serve as one, but unfaced foam needs a separate barrier.

Why a Small Gap Behind Siding Helps the Wall Dry

A rainscreen gap is a narrow air space between the siding and the water barrier. Penn State researchers found that a gap of about 1/4 inch helps water drain and lets the wall dry. Vinyl's curved shape leaves a gap on its own, but fiber cement often sits tight to the wall. Thin wood furring strips or drainage mats can add that space, at some added cost.

• Laps that run shingle-style, upper layer over lower, from the bottom of the wall up.
• Seams sealed with tape made for that barrier, as its maker calls for.
• Wrap that goes on before windows and doors, so it ties into their flashing.
• No tears, gaps or loose flaps where wires, pipes or vents pass through.

What a Siding Crew Checks in the Sheathing Underneath

Sheathing stiffens the wall and gives the siding a solid, even surface to nail into. The Vinyl Siding Institute says siding should never go straight onto bare studs. It lists plywood, OSB and foam sheathing as acceptable bases, as long as the surface is smooth and flat. OSB is a panel made from wood strands pressed and glued together. In high-wind areas, FEMA's post-storm advice calls for hanging vinyl over wood panels like plywood or OSB.

When old siding comes off, the sheathing shows how the wall has fared. A crew checks for soft or dark spots, loose panels and nails that missed the framing. Rotted sections get cut out and replaced so the new siding has a sound base. Foam sheathing adds a detail, since trim and light fixtures may need spacers to sit flush over it. Skip these checks, and new siding can wave, crack or pull loose from a weak wall.

• Soft, stained or swollen sheathing, often below windows and at the base of walls.
• Gaps or missing panels that would leave siding nails with nothing solid to grip.
• Bumps or dips in the wall that would show through and strain new panels.
• The type of sheathing, since foam, plywood and OSB each call for different details.

Flashing Behind Siding at Windows, Fixtures and Wall Changes

Flashing is thin metal or membrane that turns water back out of the wall at every break. The Vinyl Siding Institute calls for sill flashing under each window at least 9 inches wide. It should run 9 inches past each side, and head flashing on top laps past the side pieces. Lights, faucets and outlets get mounting blocks, which are flashed boxes that let siding fit snugly around them. Where siding meets a different wall finish, head flashing and a J-channel, an edge trim, go in.

On a wind-prone coast like ours, flashing around openings gets extra attention. The Building America Solution Center advises extra-long flanges plus sealant and tape in high-wind areas. Even so, FEMA notes that walls have no backup layer like a sealed roof deck. Barriers can cut down leaks, but they cannot hold off hours of storm rain on a bare wall. FEMA says keeping the siding itself firmly in place is the most effective way to limit water.

Signs of Trouble Hidden Behind Your Siding

You cannot see these layers without removing siding, but the wall leaves clues. Look inside for stains or peeling paint below windows, and swollen trim or baseboards along outside walls. Outside, watch for siding that bulges, stays damp long after rain or grows mildew in one spot. Leave the panels in place, since pulling them off can tear the wrap behind them.

• Note where stains show up inside, and which wall and window sit above them.
• Take photos after a heavy, windy rain to track whether the spots grow.
• Ask whether new siding includes new wrap and window flashing, or only panels.

When These Hidden Wall Layers Can Be Fixed or Upgraded

The layers behind siding can only be fixed or upgraded while the siding is off. That happens during a full replacement, after a storm strips a wall, or when an addition goes on. Each is a chance to add a better barrier, a rainscreen gap or wider flashing at the windows. Hanging new siding over old skips that chance, since the old wrap and sheathing stay hidden. When we install siding, the wall gets wrapped and flashed before the first panel goes up.

Siding is the part of the wall you see, while the hidden layers handle the water that slips past. Laps that run the right way, sound sheathing and well-placed flashing send that water back outside. A small drainage gap helps the wall dry out between storms. Ask about each layer before work starts, so you know what your new siding sits on. Then the wall can stay dry for years after the last panel goes up.